Leadless Multi-Layered Ceramic Capacitor Stacks

ABSTRACT

A stacked MLCC capacitor is provided wherein the capacitor stack comprises multilayered ceramic capacitors wherein each multilayered ceramic capacitor comprises first electrodes and second electrodes in an alternating stack with a dielectric between each first electrode and each adjacent second electrode. The first electrodes terminate at a first side and the second electrodes second side. A first transient liquid phase sintering conductive layer is the first side and in electrical contact with each first electrode; and a second transient liquid phase sintering conductive layer is on the second side and in electrical contact with each second electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of pending U.S. patentapplication Ser. No. 13/114,126 filed May 24, 2011 which claims priorityto expired U.S. Provisional Patent Application No. 61/348,318 filed May26, 2010 both of which are incorporated herein by reference. The presentapplication also claims priority to pending U.S. Provisional PatentApplication No. 61/729,783 filed Nov. 26, 2012 which is incorporatedherein by reference.

BACKGROUND

The present invention is related to electronic components and methods ofmaking electronic components. More specifically, the present inventionis related to electronic components and methods of making electroniccomponents, particularly stacked leadless multi-layered ceramiccapacitors with improved terminations for attachment of an external leador lead frame or for direct lead-less attachment of the electroniccomponent such that the component can subsequently be connected to anelectronic circuit by a variety of secondary attachment materials andprocesses.

In general, the method of formation of a conductive termination, andmaterials used, is critical for reliable performance. The performance inuse, when subsequently assembled in an electronic circuit, is directlyrelated to the conductive termination. Historically, lead (Pb) basedsolders have been used to attach components to electronic circuit boardsor to attach external leads to the electronic component. More recently,the use of hazardous substances in electrical and electronic equipment,as typified by the European RoHS legislation, has restricted the use oflead (Pb) in solder which has led the industry to seek variousalternatives.

U.S. Pat. No. 6,704,189, for example, describes the use of Sn basedsolder with 10-30% Sb to form a contact between external leads andplated Multi-Layer Ceramic Capacitor (MLCC) components. However, thesolders described have a liquidus below 270° C. By way of comparison,high-Pb solders such as Sn10/Pb88/Ag2 have a liquidus of about 290° C.It is generally recognized in the industry that a melting point at least30° C. above any subsequent processing temperature is desirable toinsure reliability of the external lead attachment. The ability toachieve high melting points has become critical since solders based onSn, Ag and Cu, which are referred to in the art as SAC solders, are nowbecoming the common choice for attachment in Pb-free circuits. SACsolders have to be reflowed at higher temperatures, typically about 260°C., than the older Pb-based alternatives such as Sn63/Pb37 which had amelting point of 183° C. The contact material to the external lead, orfor forming the terminal, must be capable of sustaining temperatureswell above this in order not to melt, or partially melt, which causessignificant reliability issues. A temperature of at least 30° C. abovethe melting point of the SAC solder is desired. Due to materialscompatibility and higher processing temperatures involved with thesemi-conductor technologies, gold/germanium, gold/silicon, and gold/tinalloys were developed to attach a die to substrates. Since the die andtheir mating surfaces have a low difference in thermal coefficient ofexpansion CTE, these alloys provided high temperature capabilities andhigh strengths having tensile strengths in the range of 20,000 psi andshear strengths in the range of 25,000 psi. However, these materialsalso require higher processing temperatures due to their higher meltingpoints of generally above 350° C. Their high process temperature hasprevented their wider use in electronics. Tin and indium have been addedto combinations of Zn, Al, Ge and Mg to form higher temperature leadfree solders. However, zinc and aluminum powder tend to form oxide filmson the surface which are associated with poor wettability in thesubsequent solders making them impractical to use. Solders with tin,zinc, cadmium, and aluminum are available but they are typically used intheir eutectic alloy form because their alloys, other than eutectics,have wide plastic ranges of 50-175° C. limiting their use to veryspecific applications outside of electronics. Cadmium, zinc, and silveralloy solders are good for soldering aluminum. Once the liquidustemperatures move above 450° C. the solders are referred to as brazingsolders which are typically used in structural applications rather thanelectrical applications. Methods of forming Pb-free, high temperaturebonds to capacitors that retain their integrity above 260° C. and areeconomical to manufacture have therefore yet to be realized.

The following patents describe the materials and processes of TLPS withrespect to forming a conductive bonds. U.S. Pat. No. 5,038,996 describescoating two mating surfaces one with Sn and the other with Pb andforming a joint by raising the process temperature to a temperature ofabout 183° C. which is slightly below the melting point of Sn. TransientLiquid Phase Sintering (TLPS) formulations disclosed in U.S. Pat. No.5,853,622 combine TLPS materials with cross linking polymers to create aconductive adhesive having improved electrical conductivity as a resultof intermetallic interfaces between the metal surfaces created by TLPSprocess. The spraying of two mating surfaces, with one surface having alow temperature melting material and the mating surface having acompatible higher melting temperature material forms a joint whenheating to the melting point of the lower temperature material asdiscussed in U.S. Pat. No. 5,964,395.

U.S. Pat. No. 5,221,038 describes the use of SnBi or SnIn for solderingdiscrete components such as resistors and the like to printed circuitboards using the TLPS process. The use of Ag/SnBi coated to two matingsurfaces to mount electronic modules to substrates was disclosed in U.S.Pat. No. 6,241,145. U.S. Pat. Publ. No. 2002/0092895 discusses thedeposition of materials on two mating surfaces, a substrate and thesurface of the bumps on a flip chip, elevated to a temperature to causediffusion between the materials to create a TLPS compatible alloy. U.S.Pat. Publ. No. 2006/0151871 describes the use of TLPS in formingpackages containing SiC or other semiconductor devices bonded to othercomponents or conductive surfaces. U.S. Pat. Publ. No. 2007/0152026describes the placement of TLPS compatible materials on mating surfacesfollowed by reflowing the lower melting point material and thenisothermal aging to complete the diffusion process where the two devicesto be joined are a micro-electromechanical system (MEMS) device to amicroelectronic circuit. U.S. Pat. No. 7,023,089 describes the use ofTLPS to bond heat spreaders made from copper, black diamond, or blackdiamond copper composite to silicon die. These patents and applicationsdescribe the processing of TLPS to bond components to circuit boards butdo not contain any teaching regarding their use to form terminations onelectronic components or in the attachment of components to lead frames.

In a more recent development U.S. Pat. Publ. No. 2009/0296311 describesa high temperature diffusion bonding process that welds the lead to theinner electrodes of a multi-layer ceramic component. TLPS materials areplated on the faces of mating surfaces to be joined together byintroducing heat to initiate the diffusion process. In this case,intimate mutual contact across the surfaces is required between thecomponent and lead frame to facilitate the diffusion. This limits theapplication to the joining of surfaces that can form an intimate line ofcontact and this application cannot accommodate components of differinglength connected to the lead frame. Furthermore, high temperatures inthe range of 700 to 900° C. are described to achieve a welded bond.These high formation temperatures require careful process design, suchas preheating stages, to avoid thermal shock damage to the multi-layerceramic components and even then this may not be suitable for allmaterials.

Other Pb free attachment technologies are described in the art yet noneare adequate.

Solder is an alloy consisting of two or more metals that have only onemelting point, which is always lower than that of the metal having thehighest melting point and generally has a melting point of less thanabout 310° C. depending on the alloy. Solder can be reworked, meaning itcan be reflowed multiple times, thus providing a means to remove andreplace defective components. Solders also make metallurgical bonds byforming intermetallic interfaces between the surfaces they are joining.As solders wet to their adjoining surfaces, they actually flow outwardand spread across the surface areas to be joined.

MLCC's are widely used in a variety of applications. Most typically anMLCC, or a stack of MLCC's, is mounted to a circuit board as a discretecomponent. A particular problem associated with MLCC's is theirpropensity to crack when subjected to stress such as bending of thecircuit board. To avoid these stress fractures the MLCC's are mountedbetween lead frames, such as one of each polarity, and the lead framesare then attached to the circuit board by soldering and the like. Thelead frames have been considered in the art to be a necessity and mucheffort has been spent designing lead frames capable of withstanding thestress associated with board flexure without imparting the stress on theMLCC. The lead frame design and material is particularly difficult dueto the differences in coefficient of thermal expansion and the desire tominimize equivalent series resistance (ESR), inductance and otherparasitics. In spite of the desire to eliminate the lead frame those ofskill in the art have not been able to do so since any flexure of thecircuit board transfers directly to the MLCC virtually insuring damageto the MLCC.

In spite of the ongoing, and intensive effort, the art still lacks anadequate capacitor. There is an ongoing need for lead connections withimproved reliability for high temperature applications, especially lead(Pb) free.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method forforming metallic external terminals, suitable for attachment to a leadframe or for use as a leadless capacitor stack, which can be reflowedwithout compromising the metallic external leads or lead frameattachment during subsequent assembly into an electronic circuit.

It is another object of this invention to provide an improved method forforming terminations that are suitable for the attachment of a leadframe or as a leadless termination which can withstand subsequent solderreflow process to an electronic circuit without compromising thetermination or the lead attachment interconnect.

It is another object of the invention to provide a stacked MLCC whichcan be mounted without a lead frame wherein board flexure does not causethe expected amount of stress cracking of the MLCC's.

It is another object of this invention to form terminations orinterconnects on electronic components having the advantage of lowinitial process temperatures but having high subsequent melting pointtemperatures without the use of banned materials such as lead or cadmiumor costly materials such as gold.

These and other advantages, as will be realized, are provided in acapacitor stack comprising multilayered ceramic capacitors wherein eachmultilayered ceramic capacitor comprises first electrodes and secondelectrodes in an alternating stack with a dielectric between each firstelectrode and each adjacent second electrode. The first electrodesterminate at a first side and the second electrodes second side. A firsttransient liquid phase sintering conductive layer is the first side andin electrical contact with each first electrode; and a second transientliquid phase sintering conductive layer is on the second side and inelectrical contact with each second electrode.

Yet another embodiment is provided in a method of forming an electricalcomponent comprising:

providing multilayered ceramic capacitors wherein each multilayeredceramic capacitor comprises first electrodes and second electrodes in analternating stack with a dielectric between each first electrode andadjacent second electrode of wherein said first electrodes terminate ata first side and said second electrodes terminate at a second side;stacking the multilayered ceramic capacitors such that each first sideis parallel and each said second side is parallel;forming a first layer of a first component of a transient liquid phasesintering conductive layer;forming a second layer of the first component of the transient liquidphase sintering conductive layer;contacting the first layer and the second layer with a second componentof transient liquid phase sintering conductive layer;heating to a first temperature sufficient to form a first transientliquid phase sintering conductive layer comprising the first componentand the second component wherein the first transient liquid phasesintering conductive layer is in electrical contact with the firstelectrodes and forming a second transient liquid phase sinteringconductive layer comprising the first component and the second componentwherein the second transient liquid phase sintering conductive layer isin electrical contact with the second electrodes thereby forming a stackcapacitor.

Yet another embodiment is provided in a method of forming a stack ofmultilayered ceramic capacitors comprising:

providing a multiplicity of multilayered ceramic capacitors wherein eachmultilayered ceramic capacitor comprises:first electrodes and second electrodes in an alternating stack with adielectric between each first electrode and each adjacent secondelectrode wherein the first electrodes have a first polarity andterminate at a first side of the multilayered ceramic capacitor and thesecond electrodes have a second polarity and terminate at a second sideof the multilayered ceramic capacitor;forming a stack of the multilayered ceramic capacitors;forming a first transient liquid phase sintering conductive layer inelectrical contact with first electrodes of adjacent multilayeredceramic capacitors; and forming a second transient liquid phasesintering conductive layer in electrical contact with second electrodesof adjacent multilayered ceramic capacitors.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side schematic view of an embodiment of the invention.

FIG. 2 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 3 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 4 is a side schematic view of an embodiment of the invention.

FIG. 5 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 6 is a cross-sectional schematic exploded view of an embodiment ofthe invention.

FIG. 7 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 8 is a side cross-sectional schematic view of an embodiment of theinvention.

FIG. 9 is a side cross-sectional schematic view of an embodiment of theinvention.

FIG. 10 is a cross-sectional schematic view of an embodiment of theinvention.

FIG. 11 is a side schematic view of stacked MLCC's.

FIG. 12 is a side schematic view of an embodiment of the invention.

FIG. 13 is a graphical representation of an embodiment of the invention

FIGS. 14 and 15 are electron micrographs of cross-sections of. couponsbonded in accordance with an embodiment of the invention.

FIGS. 16 and 17 are electron micrographs of cross-sections of couponsbonded in accordance with an embodiment of the invention.

FIGS. 18-21 are graphical representations illustrating advantagesprovided by the invention.

FIGS. 22 and 23 are graphical illustration of board flexure testingresults.

FIG. 24 is an electron micrograph of sheared overlap joint of twocoupons having plated surfaces.

FIG. 25 illustrates two coupons bonded with TLPS paste in accordancewith an embodiment of the invention.

FIGS. 26 and 27 are graphical illustrations of board flexure testingresults.

DETAILED DESCRIPTION

The present invention is related to an MLCC with improved bonding to anexternal lead or lead frame or for improved bonding between MLCC's in astack for use as a lead-less MLCC. More specifically, the presentinvention is related to the use of transient liquid phase sintering(TLPS) or polymer solder to form a termination to the component or toattach external leads to the component. The improved terminations havethe advantage of being able to accommodate different surface finishes aswell as components of differing lengths. Furthermore, since no solderballs are formed components can be stacked on top of each other withoutthe gaps normally required for cleaning as with solder attachmenttechnology. The TLPS or polymer solder can be directly bonded with theinner electrodes of the component and the termination can be formed atlow temperature. In a preferred embodiment higher density terminationscan be prepared by using a thermo-compression process thereby formingimproved external lead attachment bonds.

Solders are alloys which do not undergo a change in composition afterthe first reflow. Solders have only one melting point and can beremelted an indefinite number of times. The most common solder is 60%Sn40% Pb. Solders have been the materials of choice in electronics toprovide the mechanical and electrical interconnects between componentsand circuit boards or substrates. Solders are very well suited for massvolume production assembly processes. The physical properties of soldercan be altered simply by changing the ratios or the metals used tocreate a solder alloy. When solder is referenced from this point on itwill imply an alloy of at least two metals that can be remelted anindefinite number of times.

Conductive Epoxy/adhesives are typically cross linking polymers that arefilled with conductive fillers typically silver or gold flakes orparticles to create electrically conductive epoxy polymeric bonds.Unlike solders, conductive adhesives can only be cured once, and cannotbe reworked. As the metal particles touch each other they form ameandering conductive path through the epoxy to create an electricalconnection between two or more components. Conductive epoxies/adhesives,solders, and epoxy solders typically have temperature limitations ofless than 315° C.

Polymer solder is another material being used to create a metallurgicalconnection between two or more compatible metals. Polymer solder is thecombining of a solder with cross linking polymers, epoxy being a typicalexample. The solder provides the electrical conductivity and the bulk ofthe mechanical strength of the joint while the epoxy forms polymericbonds that provide additional mechanical strength and increases thetemperature capability over the solder itself.

Transient liquid phase sintering (TLPS) bonds are distinguishable fromsolders. TLPS materials are mixtures of two or more metals or metalalloys prior to exposure to elevated temperatures thereby distinguishingthe thermal history of the material. TLPS materials exhibit a lowmelting point prior to exposure to elevated temperatures, and a highermelting point following exposure to these temperatures. The initialmelting point is the result of the low temperature metal or an alloy oftwo low temperature metals. The second melting temperature is that ofthe intermetallic formed when the low temperature metal or alloy, formsa new alloy with a high temperature melting point metal thereby creatingan intermetallic having a higher melting point. TLPS materials form ametallurgical bond between the metal surfaces to be joined. Unliketin/lead or lead (Pb) free solders, the TLPS do not spread as they formthe intermetallic joint. Rework of the TLPS system is very difficult dueto the high secondary reflow temperatures. Transient Liquid PhaseSintering is the terminology given to a process to describe theresulting metallurgical condition when two or more TLPS compatiblematerials are brought in contact with one another and raised to atemperature sufficient to melt the low temperature metal. To create aTLPS process or interconnect, at least one of those metals being from afamily of metals having a low melting point such as tin (Sn), Indium(In) and the second coming from a family having high melting points suchas Copper (Cu) or Silver (Ag). When Sn and Cu are brought together andthe temperature elevated the Sn and Cu form CuSn intermetallics and theresulting melting point is higher than the melting point of the metalhaving a low melting point. In the case of In and Ag, when sufficientheat is applied to the In to cause it to melt it actually diffuses intothe Ag creating a solid solution which in turn has a higher meltingpoint than the In itself. TLPS will be used to generically reference theprocess and the TLPS compatible materials used to create a metallurgicalbond between two or more TLPS compatible metals. TLPS provides anelectrical and mechanical interconnect that can be formed at arelatively low temperature (<300 C) and having a secondary re-melttemperature >600 C. These temperatures are determined by the differentcombination of TLPS compatible metals. TLPS will be used to genericallypertain to the process and materials used to create a TLPS metallurgicalbond or interconnect.

TLPS bonds can be formed at relatively low initial process temperatures,as low as 157° C. Once the TLPS bond process is complete, the resultingjoint has a much higher melting temperature than its initial processtemperature, typically greater than 300° C. with secondary melting above450° C. common for many material sets. TLPS differs from traditionalsolders in that solders are formed by melting two or more metalstogether to create an alloy having specific properties. These propertiescan be altered simply by adding additional metals to the alloy orchanging the % composition of the metals in the alloy. The solder alloycan then be remelted and solidified to join two or more surfaces. TLPSinitially is not an alloyed material like that of solder alloys. TLPS isa metallurgical process based on the diffusion or sintering of two ormore metals into one another and occurs specifically at the interfacebetween two surfaces. Once a TLPS interface is created it cannot bere-melted at a low temperature. The higher re-melt temperature of TLPS,once the sintering or diffusion process has been completed, prohibitsre-work of the assembly in many cases since these will sustainirreparable damage at these high temperatures. The TLPS process isachieved by bringing a low temperature melting metal such as indium ortin in contact with a high temperature melting metal such as silver orcopper respectively and elevating the temperature to a point where thelow temperature metal melts and diffuses or sinters with the highmelting temperature material. The rate of diffusion or sintering is atime temperature function and is different for the differentcombinations of metals. The result is a solid solution having a new melttemperature approaching that of the high temperature melting metal.

The TLPS technology is particularly suited to providing both amechanical and electrical conductive metallurgical bond between twomating surfaces that are relatively flat. The metals typically used forthe TLPS process are selected from two metal families. One consists oflow melting temperature metals such as indium, tin, lead, antimony,bismuth, cadmium, zinc, gallium, tellurium, mercury, thallium, selenium,or polonium and a second family consisting of high temperature meltingmetals such as silver, copper, aluminum, gold, platinum, palladium,beryllium, rhodium, nickel, cobalt, iron and molybdenum to create adiffused solid solution.

It is highly desirable to use a flux free process to eliminate anypotential voids within the joint. Since TLPS is a sintering basedprocess, the bond line is uniform and void free. Fluxes, which arenecessary with solders, get entrapped in the joint gets burned out thusleaving a void. In the case with the semi-conductor industry andspecifically with die attach processes, these voids can create hot spotswithin the I/C which can lead to premature failure and reliabilityissues. TLPS addresses this issue since TLPS is a sintering process andfree of fluxes. When the two metals are mated together and heat isapplied, the lower melting metal diffuses into the higher melting metalto create a solid solution across the mating surface area. To create asolid uniform bond line, it is mandatory that the mating surfaces beflat and coplanar to ensure intimate contact across the entire matingsurface. The required flatness of the mating surfaces also limits theapplication of this technology because there are many surfaces that arenot sufficiently planar to yield a good joint.

A TLPS compatible metal particle core combined with a liquid carriermaterial to form a paste can be applied between two non-planarnon-uniform surfaces having mixed surface preparation technologies suchas plating, sintered thick film, and or plated sintered thick film andthen heating to the melting temperature of the metal having the lowestmelting point and holding that temperature for a sufficient amount oftime to form a joint. A single metal particle core eliminates the needfor multiple metals in a paste thus making the ratios of metals anon-issue. It is also possible to create a single particle by usingsilver, a metal having a high melting point of approximately 960° C., asa core particle and then coating that particle with a metal shell havinga low temperature metal such as indium having a melting point of 157° C.The advantage of using indium is that as it melts it diffuses intosilver. If this bi-metal particle of silver and indium is placed betweentwo surfaces each coated with silver, the indium will then diffuse intothe silver surfaces as well as the silver core creating a solid solutionjoint. Other metals having a low melting point such as indium that couldbe considered for this bi-metal single particle include tin, antimony,bismuth, cadmium, zinc, gallium, tellurium, mercury, thallium, selenium,polonium or lead and metals having high melting points such as silverare, copper, aluminum, gold, platinum, palladium, beryllium, rhodium,nickel, cobalt, iron and molybdenum may also be considered as possiblecombinations.

Indium powder mixed with a flux and solvent to form a paste can beapplied to produce a TLPS metallurgical bond between two coupons havinga base metal of copper overplated with Ni and then overplated with about5 microns (200μ inches) of silver. The samples can be prepared bydispensing the indium paste onto a coupon having the plated surfaces asmentioned and then placing two coupons in contact with one another andheated to 150° C. for 5 seconds, followed by increasing the temperatureto about 320° C. for about 60 seconds. The joint strength of the samplethus prepared can exhibit a pull weight in the range of 85-94 poundsequating to shear stress of 4,177 psi and a pull peel weight in therange of 5-9 pounds with an average of 7 pounds can be achieved. Theseresults are comparable to results for SnPb solders having shearstrengths of approximately 3000 psi and pull peel strengths in the 7-10pounds range. One major difference is that the AgIn joint can withstandsecondary melt temperatures exceeding 600° C. These results indicatethat the In paste used to bond two silver plated coupons is at leastequivalent if not stronger than current solder SnPb solders but also hasa much higher secondary melt temperature thus yielding a materialsuitable for high temperature interconnect applications and also beinglead free.

Methods to combine a lead frame to a structure generally consist ofcoating two mating surfaces one with a high temperature melting metaland its mating surface with a low temperature melting metal. The coatingprocess may consist of vapor deposition or plating. A second method isto sandwich a preform film made from a low melting point metal or analloy of two or more low melting point metals between two planarsurfaces coated with a high melting point metal such as Ag, Cu, or Au. Athird method is to create a paste consisting of particles of a highmelting point metal such as copper and then adding particles of twoalloyed low melting point metals such as Sn—Bi and mixed into a dualpurpose liquid that cleans the surfaces to be bonded and also serves asthe liquid ingredient to the metal particles to form a paste mixture.

If full diffusion of the two metals is not complete in the stated cycletime and the maximum secondary reflow temperature is not reached, thejoint can be subjected to a second heating process. In this case thejoint, or assembly, can be subjected to a temperature higher than thatof the low melting point material and held for a period of time from 15minutes up to 2 hours. The time and temperature can be varied to providea desirable secondary reflow temperature as dictated by secondaryassembly processes or final environmental application requirements. Inthe case of the indium/silver TLPS, secondary melt temperatures inexcess of 600° C. can be achieved.

A joint can be formed by subjecting the assembly to a temperaturesufficient to melt the low temperature metal for a time sufficient tocreate a mechanical joint such as for 5 seconds to 30 seconds. Thenduring a secondary heating process, the joint is subjected to atemperature and time sufficient to allow the indium and silver todiffuse thereby creating an alloy having a higher secondary reflowtemperature.

In addition to applying a paste to form a TLPS alloy joint betweensuitable surfaces this can also be achieved with a preform. In itssimplest manifestation the preform can be a thin foil of the lowtemperature TLPS component. Alternatively the preform can be produced bycasting and drying the paste to remove the solvent. The resulting solidpreform can be placed between the surfaces to be bonded. In this case itmay be necessary to add a suitable binder to the paste for additionalstrength after drying. In all these cases the preform should bemalleable such that it can conform to the surfaces to be bonded.

An interconnect comprising a single metal, such as indium, containedwithin a paste can be used to form a bond to a surface coated with ahigh melting point metal, such as silver. The diffusion of the indiuminto silver allows a lower temperature transient liquid phase to formthat subsequently reacts to achieve a higher temperature bond. Achievinga high rate of diffusion in the lower melting point paste is critical tothis bond formation. In order to achieve the desired properties in thefinal joint, such as reduced voids and a homogeneous phase the additionof other metals to the paste may be desirable. However, it is criticalto retain the high diffusivity of the low melting point material. Forthis reason if one or more metals are required in addition to the lowmelting point metal it is preferred that these be incorporated bycoating the metal powders prior to forming the paste. Coating the lowestmelting point metal onto the higher melting point metal is preferred toretain an active surface. Coatings also have the desired effect ofreducing the diffusion lengths between the different metallic elementsof the paste allowing preferred phases to be more readily formed asopposed to a simple mixing of one or more additional metal powders tothe single metal paste.

It is preferable that alloys be excluded. Alloys reduce the diffusionactivity of the paste. The coated metal powders can preferably be formedusing plating prior to incorporation within the paste.

Conductive adhesives are typically cross linking polymers filled withsilver or gold particles that cure or cross link within a specifiedtemperature range, generally 150° C., to form a mechanical bond to thematerials to be joined. Their conductivity is created by the metalparticles making intimate contact with one another, within the confinesof the polymer matrix, to form an electrically conductive path from oneparticle to another. Because the binder is organic in nature, they haverelatively low temperature capabilities, normally in the range of about150° C. to about 300° C. Conductive epoxies, once cured, cannot bereworked. Unlike TLPS bonds, exposure to high heat or corrosiveenvironments may decompose the polymeric bonds and oxidize the metalparticles degrading the electrical properties. Both the electrical andmechanical performance of the interconnect can be compromised resultingin increased ESR and decreased mechanical strength.

Polymer solders may comprise conventional solder systems based on Pb/Snalloy systems or lead free systems, such as Sn/Sb, which are combinedwith crosslinking polymers which serve as cleaning agents. Thecross-linked polymers also have the ability to form a cross-linkedpolymer bond, such as an epoxy bond, that forms during the melting phaseof the metals thereby forming a solder alloy and a mechanical polymericbond. An advantage of polymer solders is that the polymeric bondprovides additional mechanical bond strength at temperatures above themelting point of the solder, thus giving the solder joint a higheroperating temperature in the range of about 5 to 80° C. above themelting point of the solder. Polymer solders combine current solderalloys with a cross linking polymer within the same paste to provideboth a metallurgical bond and a mechanical bond when cured, such as byheating, to provide additional solder joint strength at elevatedtemperatures. However, the upper temperature limits and joint strengthhas been increased, just by the physical properties of the materials. Apractical limit of 300° C. remains whereas the bonds created by TLPS canachieve higher temperatures.

In one embodiment the polymer solder bond may be complimentary wherein,instead of the preferred mixture of polymer and solder, the polymerforming components and metallurgical bond forming components may beseparate with both between the faces to be mated.

A TLPS or polymer solder paste can form a termination on an electroniccomponent that can subsequently be attached to an electronic circuit byother methods and/or materials. A metallurgical intermetallic bond isformed that can be lead (Pb) free which has improved joint strengthcompared to other material types such as Pb-free solder at elevatedtemperatures. The TLPS or polymer solder joint may be made directly witha buried electrode or electrodes within the component or through othermaterials in contact with these electrodes. The TLPS or polymer solderjoint does not have to overlap the edge of the component.

The use of TLPS or a polymer solder in paste form allows uneven surfacesto be joined. More specifically, the use of TLPS or polymer solder inpaste form allows two irregular shaped surfaces to be joined with nointimate, or continuous, line of contact. This is particularlyadvantageous compared to plated surfaces that are subsequently diffusionbonded where the surfaces have to be in an intimate continuous line ofcontact during this process. This also allows electronic components ofdiffering lengths to be combined within a stack or stacked within a leadframe. Since TLPS does not form solder balls, the stacked components canbe placed on top of one another with the terminations in the sameorientation with no gaps required for cleaning as needed in conventionalattachment using solder.

Since the TLPS or polymer solder pastes do not flow like a conventionalsolder there is no need to employ solder dams on the lead frames. Thisfeature provides a significant manufacturing convenience.

The TLPS or polymer solder paste can be used to form bonds between 2 ormore components to each other or within a common lead frame. In the caseof the lead frame, components of different lengths can be attached andthere is no need for gaps between the components to clean solder ballssince these do not occur. The resulting stacks are therefore thinnerthan if assembled with conventional solder. Polymer solders minimizesolder balls and TLPS eliminates solder balls.

With TLPS paste thermo-compression bonding can be used to increasedensities in the bond thereby forming more reliable joints than whenrelying on temperature alone. Both the mechanical properties andelectrical properties are improved by thermo-compression bonding.

TLPS can be used to form a bond directly to the inner electrode of theelectronic component. In MLCC's the inner electrode can be a highmelting point metal. A low melting point metal can be coated on the edgeof the MLCC and a high melting point metal layer, such as in a sheet orcoupon, exterior thereto. Upon heating the low melting point metal canalloy with the internal electrode and the exterior metal thereby forminga metallurgical bond directly to the inner electrode.

It is particularly preferred that a low temperature be used to form aninitial bond between the transient liquid phase sintering conductiveadhesive and a component. Formation of the initial bond is followed byisothermal aging to generate a high temperature joint capable ofsustaining higher temperatures. The reflow temperatures occur duringattachment of the component to a circuit using a secondary attachmentprocess and are less than the melt temperature of the highest meltingelement and the melt temperature of the alloy formed during the heatingto form the initial bond. This is favorable compared to SAC type solderthat requires a reflow of about 260° C.

A two-step reflow can also be used with the transient liquid phasesintering process wherein in the first step an electrically conductivemetallurgical bond is formed at low temperature using a relatively shorttime cycle, in the range of 5 seconds to 5 minutes, and low temperature,in the range of 180° C. to 280° C., depending on the metals being usedin the TLPS alloying process. In the second step the part is subjectedto an isothermal aging process using a temperature range of 200° C. to300° C. for a longer duration such as, but not limited to, 5 minutes to60 minutes. The shorter times required to form the initial bond are wellsuited for an automated process. In another method a single step processcan be used wherein the TLPS forms a terminal, or conductivemetallurgical bond, between the external leads and electroniccomponent(s) at temperatures of, for example, 250° C. to 325° C. for aduration of, for example, 10 seconds to 30 seconds. Lower temperatures,such as 175° C. to 210° C., can be used for a longer duration, such as10 to 30 minutes. This is particularly useful when the electroniccomponent itself is sensitive to temperature.

Typically, the terminations are formed, preferably by heating, using aone-step sintering process making an electrically conductivemetallurgical bond at a temperature range of, but not limited to, 190°C. to 220° C., for a time of, but not limited to, 10 minutes to 30minutes. It is most preferred that the second melt temperature exceedthe first melt temperature by at least 80° C. The metallic bond can havea second melting temperature in excess of 450° C., thus making thistechnology a viable option for a low temperature processing lead (Pb)free solution suitable for use in subsequent high temperatureapplications. However, this type of process is more suited for batchtype processes typical of semiconductor processing and some PCBprocessing but it is not conducive for high volume in-line terminationand external lead attachment for electronic components includingmulti-layer ceramic capacitors. Furthermore, processing the TLPS thisway can result in a high degree of porosity particularly with highlevels of organic content.

TLPS materials can be processed using a two-step process to achievefavorable interconnected joints. The first step forms a robustelectrically conductive metallurgical joint in a relatively shortprocess time of 30 seconds or less at a temperature in the range of 225°C. to 300° C. The second step is a sintering step that subjects theparts to a temperature of 200° C. to 250° C., or less, for a time of 5minutes to 30 minutes to complete the alloying process. The two-stepprocess is satisfactory for high volume in-line assembly where asubsequent batch sintering process is acceptable. However, as with theaforementioned single step process the porosity is often undesirablyhigh.

In many applications a high degree of porosity may be acceptable.However, in harsh environments, such a high humidity or in circuit boardmounting processes, high porosity is not desirable since water or otherchemicals may penetrate through the bond which may cause the componentto fail. A preferred embodiment of this invention is therefore to form alow porosity termination within the transient liquid phase sinteringjoint using a thermo-compression bonding process. This process has theadded advantage of using a low process time of 15 to 30 seconds at atemperature in the range of 225° C. to 300° C. in a single step makingit suitable for automation. Robust joints can be created for theapplication of attaching external leads to MLCC's using with a one-steplow temperature in less than 30 seconds and in combination withthermo-compression bonding.

Thermo compression bonding is also a preferred processing method whenusing polymer solder because it assists in the formation of ahigh-density metallurgical bond between the contacting surfaces. Theadvantages of thermo-compression include a more robust bond with respectto secondary attachment processes and attachments with higher strengthare achieved. A compressive force of 0.5 to 4.5 Kilograms/cm² (7.1 to 64psi) and more preferably 0.6 to 0.8 Kilograms/cm² (8.5 to 11 psi) issufficient for demonstration of the thermo-compression teachings herein.About 0.63 Kilograms/cm² (9 psi) is a particularly suitable pressure fordemonstration of the teachings.

TLPS comprise high temperature materials selected from copper, silver,aluminum, gold, platinum, palladium, beryllium, rhodium, nickel, cobalt,iron and molybdenum or a mixture or any combination thereof are suitablefor use in the TLPS process. The lead (Pb) free TLPS materialspreferably use either silver or copper as the high temperature componentand indium, tin, or bismuth as the low temperature component.

TLPS further comprises low temperature materials selected from tin,antimony, bismuth, cadmium, zinc, gallium, indium, tellurium, mercury,thallium, selenium, or polonium, or a mixture or an alloy of any two ormore of these.

The TLPS materials are compatible with surface finishes containingsilver, tin, gold, copper, platinum, palladium, nickel, or combinationsthereof, either as lead frame finishes, component connections or innerelectrodes to form an electronically conductive metallurgical bondbetween two surfaces. Suitable external lead or lead frame materialsinclude phosphor bronze, copper, alloys of copper such as but notlimited to beryllium copper, Cu194 and Cu192, as well as lead framesconsisting of ferrous alloys such as but not limited to Alloy 42 andKovar.

Heating can be done by any method known in the art with convectionheating, radiant heating and induction heating being most preferred.

The invention will be described with reference to the figures forming anintegral, non-limiting, component of the disclosure. Throughout thevarious figures similar elements will be numbered accordingly.

An embodiment of the invention will be described with reference to theschematic cross-sectional side view in FIG. 1. In FIG. 1, electroniccomponents, 1, such as an MLCC's, comprise external terminations, 2,which are in integral electrical contact with the internal electrodesthrough a TLPS bond as will be more readily understood from furtherdiscussions. A particular advantage is the ability to bond a stack ofMLCCs with a TLPS bonded external termination in electrical contact witheach end thereby forming a leadless stack that can be attached tocontact pads, 4, on an electronic circuit board substrate, 5, using asecondary attachment material, 3. It can be seen that in this wayelectronic components with many terminations formed with TLPS or polymersolder can be attached to a circuit without a lead. However, the highersecondary melting temperature of the TLPS joint makes TLPS preferredover the polymer solder since this allows a wider range of secondaryattachment materials to be considered. Element 1A may be an additionalelectronic component as described above or it may represent asacrificial chip which absorbs flexure. The sacrificial chip ispreferably sufficiently large to absorb flexure but no larger thannecessary. A sacrificial chip with a thickness of 35-60 thousands of aninch is sufficient.

A leadless stack of MLCC's is illustrated in schematic cross-sectionalside schematic view in FIG. 2. In FIG. 2 the leadless stack is formed byapplying and reacting a suitable TLPS paste or preform, 18, betweenexternal terminations, 7, on adjacent MLCC's. The preform, as describedherein, is preferably malleable thereby allowing the preform to conformto the adjacent surfaces. When heated the low melting metal of thepreform diffuses into the external terminations thereby forming ametallurgical bond. The preform may also contain a high melting pointmetal, preferable the same metal as in the external termination, andwhen heated the low melting point metal diffuses into the high meltingpoint metal of the preform thereby forming a metallurgical bond betweenthe high melting point metal of the preform and the external terminationto insure adequate electrical conductivity between the externalterminations of adjacent MLCC's. The external terminations can be formedby co-firing a thick film paste, or by curing a conductive adhesive,thereby forming an electrical contact with the inner electrodes, 9 and10. A dielectric, 11, separates the inner electrodes.

The external termination may comprise multiple layers to facilitate TLPSbonding. An embodiment of the invention is illustrated in schematiccross-sectional view in FIG. 3. In FIG. 3, the external terminationcomprises multiple layers with the first layer, 7′, being in directcontact with the internal electrodes, 9 and 10, of the MLCC and atermination layer, 7, being compatible with the preform, 18, and capableof forming a TLPS bond therewith. Either a solder layer or plated layer,26, can be provided which encases the termination layer and preform tofacilitate secondary attachment to the circuit board and specifically toimprove solderability.

Leadless stacks of MLCCs as described were previously consideredunavailable in the prior art unless a compliant bond was formed betweenthe components as well as forming a compliant electrical bond betweentheir respective terminations. In the inventive examples describedherein the TLPS joint serves as both an electrical and mechanical bond.The electrical bonds in the prior art have limited capability inpractice due to conductive adhesives that are limited in use to thedecomposition temperatures of the polymers, which are typically epoxy.It is usually not possible to use solders to combine MLCCs alone withina leadless stack since these are prone to reflow compromising theelectrical integrity of the stack during the secondary attachment to thecircuit board. The use of leads in the prior art to overcome some ofthese issues is limited by the inherent propensity for failures due tostress from the circuit board being transferred directly to the MLCCduring thermal cycling or thermal shock events. Combinations of leadmaterials and MLCC types to reduce these stress issues for reliableperformance have been developed but this result in other limitations.For example, Alloy 42 has been used to reduce CTE mismatch but theresult is an undesirably higher ESR for the resulting stack. Theleadless stacks manufactured using TLPS to bond MLCC with fired andplated terminations have a high resistance to board flexure crackswherein the performance is no worse than the performance of individualcapacitors thus confirming the robustness of the inventive joint withrespect to their mechanical performance. Since the TLPS forms acontinuous conductive layer between the MLCC's there is no increase inESR as mentioned for the aforementioned leaded stacks made with alloy 42leads. Since the leadless stacks do not require the stand-off associatedwith a stack the same number and type of MLCC can be formed in shorterstacks. Shorter stacks combined with the relatively low formationtemperature of the TLPS bond allows other components and additionalcircuits to be added to the stack. In cases where they are used in verymechanically demanding applications a mechanical absorption componentwith no electrical functionality can be added to the bottom of thestack.

The use of the TLPS or polymer solder termination to form a conductivebond to an external lead is shown in FIG. 4 wherein the electroniccomponent, 1, is connected to an external lead or lead frame, 6, with aTLPS or polymer solder, 8, between the external lead frame and externaltermination, 7.

In FIG. 5, TLPS or polymer solder external terminals, 12, are in directcontact with the inner electrodes, 9 and 10, of a multi-layer ceramiccapacitor. The interleaved planer inner electrodes of alternatingpolarity are separated by a dielectric, 11, and alternating innerelectrodes are in direct contact with opposing external terminations,12, formed by the TLPS or polymer solder. This embodiment provides theadditional benefit of avoiding the processing costs associated withforming other connecting materials on the electronic component. WithTLPS external terminations the embodiment of FIG. 5 can then be stackedwith similar embodiments with a TLPS bond forming a unified externaltermination thereby providing a lead-less stack of MLCC's areillustrated in FIG. 1.

An embodiment of the invention is illustrated in explodedcross-sectional schematic exploded view in FIG. 6. In FIG. 6, MLCCmonolith's comprising alternating layers of inner electrodes, 9 and 10,separated by a dielectric, 11, are illustrated wherein adjacent internalelectrodes terminate at opposite ends. In one embodiment the internalelectrodes are the high melting metal of the TLPS bond. A preform, 302,is applied to the edge of the monolith to form an external termination.The preform comprises a core which may be a layered structure, with ahigh melting metal, 303, at the interface with a low melting metal, 304.The MLCC's are stacked and the preform attached to the MLCC's whereinthe low melting metal, 304, diffuses into the internal electrodes andinto the high melting metal, 303, thereby forming a metallurgical bondbetween the preform and internal electrodes of each MLCC. The highmelting metal of the preform may form a lead frame. In anotherembodiment the low melting metal may be on the MLCC.

During the formation of the TLPS bonds the diffusion of the low meltingpoint metal depends on its reactivity as well as the time andtemperature of the bond formation process. To achieve a high reactivityit is desirable to avoid alloys but the respective metals and theirthicknesses should be chosen to prohibit the possibility of secondaryphase formation with reference to the respective phase diagrams.

An embodiment with a single MLCC is illustrated in FIG. 7. In FIG. 7TLPS or polymer solder terminations, 14, bond an external lead, 15, tothe inner electrodes, 9 and 10, and do not extend past the edge, 17, ofthe multi-layer ceramic capacitor body, 16. This embodiment reduces thefailures that occur at this overlap area since the mechanical stressesare eliminated.

In FIG. 8 a cross-section of an electronic component is shown, in thiscase a multi-layer ceramic capacitor, 100, with TLPS or polymer solderterminations, 102, contacts through a conductive interconnect toexternal leads, 104. The edge, 106, has no continuous intimate line ofcontact between the conductive connection and the external leads. Aparticular advantage is that the two surfaces of the external lead andconductive interconnect when mated together are not required to form acontinuous line of intimate contact.

An embodiment of the invention is illustrated in FIG. 9. In FIG. 9, twoMLCC's, 200 and 200′, between lead frames, 206, are illustrated for thepurposes of discussion with the understanding that many could bestacked. Each MLCC has a TLPS or polymer solder terminations, 202, whichcovers only a portion of the edges, 108, of the MLCC. This allows theMLCC's to be closely spaced with a minimum, or no gap, between the facesof the MLCC. The TLPS or polymer solder terminations, 202, can bepreformed as a mixture or layered structure and inserted between thecomponents followed by a single heating step or multiple heating steps.Alternatively, an external termination can be formed in directelectrical contact with the internal electrodes.

Diffusion driven bonding processes have been successfully used betweenflat surfaces, such as those found in die attachment, however there areapplications where creating such flat surfaces is not practicable. Inthese cases, there is a need for a high temperature solution that canaccommodate the joining of non-uniform mating surfaces having thecapability to fill gaps and voids between the mating surfaces to bejoined. Metals commonly used for TLPS technology are chosen from twometal families. One family consists of metals having a low melting pointand the second family consists of metals having a high melting point.When a member of the low temperature family is brought into contact witha member of the high temperature family and exposed to heat, the lowermelting point metal diffuses or sinters into the high temperaturemelting point metal creating an alloy having a melting point less thanthat of the high temperature material. This process referred to asTransient Liquid Phase Sintering (TLPS) makes it possible to createinterconnects at relatively low temperatures but yet have high secondaryreflow temperatures lower than that of the high melting point metal, dueto the formation of the TLPS solid solution.

An embodiment of the invention is illustrated in FIG. 10 as a schematicside view of stacked MLCC's wherein the two MLCC's, 20 and 21, havedifferent widths. The TLPS or polymer solder terminations, 22, canaccommodate electronic components of differing lengths with adequatecontact to the external leads, 23. In this way, components of differinglengths up to 2.54 mm (0.10 inches) can be attached within the samestack even though it is preferable that the lengths differ by no morethan 0.254 mm (0.010 inches). It is often desirable to join multiplenon-uniform surfaces, having a mixed technology of surface metals suchas plated silver, sintered silver or other combinations TLPS compatiblemetals. As illustrated in FIG. 10, one surface to be joined can beelectroplated, such as with silver, and the mating surface can becovered with a thick film silver paste then sintered. A single componentlow temperature metal such as indium, such as in a paste form, can thenbe deposited between the two surfaces that are to be joined each havinga silver coating or other compatible TLPS high temperature metal. Thepaste has the capability to fill the gaps between the non-uniformsurfaces of different sized MLCC's. The assembly is then heated to themelting point of indium such as at 157° C. or another suitable lowtemperature material other than indium and held at the liquidoustemperature for a period of time from 5 seconds to 15 minutes, cooled,and then allowed to solidify. The resulting joint interconnect materialwill have a secondary reflow temperature greater than the temperature ofthe low temperature material. An optional insulator layer, 70, betweenadjacent MLCC's in a stack provides protection against arcing betweenexternal leads. An insulator layer is more preferable for higher voltageapplications, such as above 200 volts, or above 250 volts. The insulatorlayer can be provided by a variety of polymeric conformal coatings basedon various chemical families which can include acrylic, polyurethane,polyimide, epoxy, parylene (paraxylylene) and silicone. The TLPS orpolymer solder terminations may comprise inert fillers, 112.

The TLPS paste or preform may have inert fillers therein to serve twopurposes. One purpose is to minimize the cost due to expensive metalsand the second purpose is to made direct electrical and metallurgicalbonds directly to the non-terminated ends of the MLCC and exposedinternal electrodes. The cost can be reduced, particularly, when a gapis to be filled as discussed relative to FIG. 10 by replacing a portionof, particularly, the high melting metal component with an inertmaterial or with a lower cost conductive material. Particularlypreferred fillers for use in place of the high melting point metal arenon-metals such as ceramics with melting points >300° C. and glasses orhigh temperature polymers with glass transition temperatures(T_(g)) >200° C. An example would be a thermosetting polymers such aspolyimide. Two particular advantages of replacing the high melting pointmetal with one of these non-metals is that the active low melting pointmetal of the TLPS with not be consumed by diffusion during the TLPS bondformation. The second advantage of inert fillers when selected from afamily of glasses having low melting points is that the glass within themixture of the TLPS paste or preform will create a bond with the exposedglass frit of the non-terminated and exposed ceramic body of the MLCC.The non-metals can also be coated with the low melting point metal bymethods such a spraying or plating.

FIG. 11 illustrates a side schematic view of a stack of two components,30 and 31, attached to external leads 32 and 33 using a conventionalsolder 34. In this case a gap, G, of at least 0.254 mm (0.010″) can berequired between the components to allow for post assembly cleaning forremoval of solder balls.

FIG. 12 illustrates, in cross-sectional side schematic view, anembodiment of this invention with a stack of two components, 30 and 31,attached to external leads, 32 and 33, using a TLPS or polymer solder,35. In this case a gap of less than 0.254 mm, and preferably no gap, canbe used between the components since no solder balls are formed andtherefore cleaning is not required. Elimination of the gap allows anoverall reduction in height of the stack thereby reducing the verticalspace required for the electronic components. Furthermore, for stacks ofmore than two components the savings in space will be even greater.

It is highly desirable to create a joint with minimum porosity thatexhibits the following characteristics: strong mechanical strength inexcess of 5 Lbs./inch for Pull Peel test, Tensile, and Shear highelectrical conductivity, low initial process temperature in the range of150° C. to 225° C., a secondary reflow temperature in excess of 300° C.or higher, between non uniform surfaces making intimate contact orhaving gaps up to 0.015 inches.

EXAMPLES

The slump test is based on a visible observation, preferably withmagnification, wherein the part is inspected after treatment to see ifthe MLCC has moved, or slumped, within the lead frame. Slumpingindicates that the reflow process has caused the integrity of the bondto the lead frame to be compromised. A movement of the MLCC within thelead frame or a visual indication of a loss of bond integrity indicatesa failure.

Example 1 Improved Mechanical Robustness of Polymer Solder

Sixty-eight identical stacks each having 2 MLCC's with a case size of5.6 mm×5.1 mm (0.22×0.20 inches) mounted in a common lead frame weremanufactured. The stacks were separated into two equal sets of 34 each.One set was a control set wherein the lead frame was attached to eachMLCC using 1 mg of Sn/Sb solder with 91.5 wt % Sn and 8.5 wt % Sb. Thesecond set was an inventive set wherein a lead frame was attached toeach MLCC using 1 mg of Sn/Sb polymer solder with 91.5 wt % Sn and 8.5wt % Sb available from Henkel as 10048-11A polymer solder. Eachcomponent was passed through a solder reflow oven at 260° C. three timesand the part examined after each pass to determine the number of chipsslumped. The results are provided in Table 1 wherein the cumulativenumber of failed parts is recorded after each pass.

TABLE 1 Adhesive type Pass #1 Pass#2 Pass#3 Control 4 5 6 Inventive 0 00

The results in Table 1 indicate that, for the control, 4 parts failed inthe first pass and one additional part failed in subsequent passeswhereas none of the inventive samples failed. The polymer solder istherefore added additional mechanical strength at elevated temperaturecompared to the solder of the control samples.

Example 2 Improved Mechanical Robustness of TLPS

Similar stacks were manufactured with silver or tin plated lead framesand attached with a Cu-based transient liquid phase sintering adhesiveavailable as Ormet 328. The samples did not exhibit any slumping orexternal lead detachment. A load test was then conducted as described inU.S. Pat. No. 6,704,189 wherein the stacks were placed in an oven with a30 g weight attached to the MLCC and suspended below the stack. Thetemperature was increased above 260° C. in steps of at least 10° C. witha 10 minute dwell at each temperature. The parts were then examined forslumping and or external lead detachment failures. In the case of silverplated external lead frames failures were detected at 360° C. but for Snplated lead frames the first failures were detected at 630° C.demonstrating a superior high temperature mechanical performance forTLPS.

Example 3 Temperature Capability of Polymer Solder

One hundred and twenty J-lead style stacks were manufactured usingidentical MLCC's, identical J-leads and thermo-compression bondingprocess. The samples were split into groups of 30 and each bonded usingvarious volumes of 91.5/8.5 Sn/Sb solder, available as Henkel 92ADA1OODAP85V EU 2460, for the control samples and polymer solder, availableas Henkel 20048-11A, as the inventive samples containing the same soldercomposition. The samples were then sent through various soldering ovensover three passes and at different temperatures. The samples were thenassessed for part slumping. The results are shown in FIG. 13. Noslumping was detected in the polymer solder samples indicating improvedhigh temperature robustness with the tested range. Polymer solder willnot withstand temperatures above 350° C.

Example 4 Durability of Polymer Solder to High Speed Secondary AssemblyProcesses

J-lead style stacks were manufactured using identical MLCC's, identicalJ-leads and a thermo-compression bonding process. Controls were preparedusing 91.5/8.5 Sn/Sb solder available as Henkel 92ADA100DAP85V EU 2460.Inventive samples were prepared using a polymer solder containing thesame solder composition available as Henkel 20048-11A. The samples weresubsequently assembled onto FR4 boards with a standard solder and sentthrough an IR reflow oven using a faster temperature ramp rate thanrecommended for the soldered lead frames. The samples were examined forslumping or lead frame contact failure. The samples containing the Sn/Sbsolder had 9 failures of 15 samples whereas the polymer solder had 0failures of 15 samples demonstrating the increased robustness withrespect to high speed assembly. The parts were subjected to the samehigh-speed assembly.

Example 5 Thermo-Compression Bonding

FIGS. 14 and 15 are photomicrographs demonstrating the bonds achievedusing TLPS Ag/Sn/Bi, available as Ormet 701 Series, and Cu/Sn/Bi,available as Ormet 280 CE Series, to bond between coupons of Ag platedphosphor bronze using an IR reflow process. Significant areas of voidsare present. FIG. 16 is a photomicrograph showing the TLPS Cu/Sn/Biafter a thermo-compression bonding process and FIG. 17 is aphotomicrograph showing the Cu/Sn/Bi after a thermo-compression bondingprocess. In both instances a dense microstructure is observed.Thermo-compression can be achieved very quickly such as in less than 5minutes with 2-10 pounds of compression.

Coupons were prepared in an analogous fashion to Example 4. A 30 gweight was suspended from the device thereby placing a stress on thethermo-compression bond. The bond was subjected to increasingtemperatures. No failures were observed even with heating up to 850° C.

On observation, with lead attachments using Cu/Sn/Bi TLPS available asOrmet 701 and 10/88/2 Sn/Pb/Ag solder the TLPS remains where it wasdeposited whereas solder flows on heating. Solder requires the use ofsolder dams and resists when used with external lead attachment whereasTLPS does not. This provides a significant manufacturing advantage.

Ormet 701 Cu/Sn/Bi TLPS to bond matte plated Sn Phosphor bronze couponsusing thermo-compression bonding at various conditions with and withoutpost curing. These results are compared to a 91.5Sn/8.5Sb solder. InFIG. 18, Sample A1 was heated at 180° C., for 20 seconds with no postsinter, Sample B1 was heated at 180° C. for 15 sec. with a 20 minutepost sinter at 210° C. Sample C1 was heated at 180° C. for 20 sec. witha 30 minute post sinter at 210° C. Sample D1 was heated at 190° C. for20 sec. with no post sinter. Sample E1 was heated at 190° C. for 20 sec.with a 15 minute post sinter at 210° C. Sample F1 was heated at 190° C.for 20 sec. with a 30 minute post sinter at 210° C. Sample G1 was heatedat 200° C. for 20 sec. with no post sinter. Sample H1 was heated at 200°C. for 20 sec. with a 15 minute post inter at 210° C. Sample I1 washeated at 200° C. for 20 sec. with a 30 minute post at 210° C. Sample J1was heated at 200° C. for 10 sec. with no post sinter. Sample K1 washeated at 230° C. for 10 sec. with no post sinter. Sample L1 was heatedat 210° C., mimicking a post sinter, for 30 minutes using 91.5Sn/8.5Sbsolder. This examples demonstrates that an initial bond can be made atlower temperatures and the bond strength can be increased significantlyby post sintering.

Example 6

A set of experiments, similar to Example 5, was performed using Ormet280CE Ag/Sn/Bi on a silver plated coupon. The results are provided in abar graph in FIG. 19. In the examples the external lead exhibits a shearstrength, measured as peak pull (in Kg) to failure, which exceeds thesolder even though no post cure was used in the thermo-compressionprocess. In each case the samples were heated at a first temperature fora first period consistent with a preheat, then the temperature wasramped to a second temperature in three seconds and the samples wereheld at the second temperature for a period of time. In FIG. 19 SampleA2 was preheated at 140° C. for 10 sec., the temperature was ramped to300° C. and held for 20 sec. Sample B2 was preheated at 140° C. for 10sec., the temperature was ramped to 300° C. and held for 10 sec. SampleC2 was preheated at 140° C. for 10 sec., the temperature was ramped to300° C. and held for 5 sec. Sample D2 was preheated at 140° C. for 3sec., the temperature was ramped to 300° C. and held for 20 sec. SampleE2 was preheated at 140° C. for 3 sec., the temperature was ramped to300° C. and held for 10 sec. Sample F2 was preheated at 140° C. for 3sec., the temperature was ramped to 300° C. and held for 5 sec. SampleG2 was preheated at 140° C. for 10 sec., the temperature was ramped to280° C. and held for 20 sec. Sample H2 was preheated at 140° C. for 10sec., the temperature was ramped to 280° C. and held for 10 sec. SampleI2 was preheated at 140° C. for 10 sec., the temperature was ramped to280° C. and held for 5 sec. Sample J2 was preheated at 140° C. for 3sec., the temperature was ramped to 280° C. and held for 20 sec. SampleK2 was preheated at 140° C. for 3 sec., the temperature was ramped to280° C. and held for 10 sec. Sample L2 was preheated at 140° C. for 3sec., the temperature was ramped to 280° C. and held for 5 sec. SampleM2 was preheated at 140° C. for 10 sec., the temperature was ramped to260° C. and held 20 sec. Sample N2 was preheated at 140° C. for 10 sec,ramped to 260° C. and held for 10 sec. Sample O2 was preheated at 140°C. for 10 sec., the temperature was ramped to 260° C. and held for 5sec. Sample P2 was preheated at 140° C. for 3 sec., the t temperaturewas ramped to 260° C. and held for 20 sec. Sample Q2 was preheated at140° C. for 3 sec., the temperature was ramped to 260° C. and held for10 sec. Sample R2 was preheated at 140° C. for 3 sec., the temperaturewas ramped to 260° C. and held for 5 sec. Sample S2 was preheated at140° C. for 10 sec., ramped to 240° C. and held for 20 sec. Sample T2was preheated at 140° C. for 10 sec., the temperature was ramped to 240°C. and held for 10 sec. Sample U2 was preheated at 140° C. for 10 sec.,the temperature was ramped to 240° C. and held for 5 sec. Sample V2 waspreheated at 140° C. for 3 sec., the temperature was ramped to 240° C.and held for 20 sec. Sample W2 was preheated at 140° C. for 3 sec., thetemperature was ramped to 240° C. and held for 10 sec. Sample X2 waspreheated at 140° C. for 3 sec., the temperature was ramped to 240° C.and held for 5 sec. Sample Y2 was preheated at 140° C. for 10 sec., thetemperature was ramped to 220° C. and held for 20 sec. Sample Z2 waspreheated at 140° C. for 10 sec., the temperature was ramped to 220° C.and held for 10 sec. Sample AA2 was preheated at 140° C. for 10 sec.,the temperature was ramped to 220° C. and held for 5 sec. Sample BB2 waspreheated at 140° C. for 3 sec., the temperature was ramped to 220° C.and held for 20 sec. Sample CC2 was preheated at 140° C. for 3 sec., thetemperature was ramped to 220° C. and held for 10 sec. Sample DD2 waspreheated at 140° C. for 3 sec., the temperature was ramped to 220° C.and held for 5 sec. The results of Example 6 demonstrate the timetemperature effect on the TLPS process without post sintering.

Example 7 TLPS Termination

TLPS Cu/Sn/Bi, available as Ormet 701, was cured onto nickel base metalelectrode MLCC's to form a termination directly to the nickel innerelectrodes. The average capacitance was 0.32 μF similar to that forstandard high fire termination materials indicating a bond with acontinuous conductive pathway had been formed to the inner electrodes.

Example 8 Temperature Durability Test

To test the strength of the adhesive bond a load test was done inaccordance with U.S. Pat. No. 6,704,189 wherein the externally leadedpart is suspended in air with a 30 gram weight attached to the bottomexternal lead. The suspended part and weight are subjected to increasingtemperatures until failure is detected by detachment of the externallead wires. The results are presented in FIG. 20 wherein the inventivesample utilizing polymer solder demonstrates a significantly better bondstrength, as a function of temperature, than the control using88Pb/10Sn/2Ag solder. In FIG. 20, the control was bonded using88Pb/10Sn/2Ag solder. In Example 1 a nickel/tin lead was bonded withconductive adhesive. Example 2 was bonded to a nickel/gold lead usingconductive adhesive. Example 3 was bonded to a nickel/silver lead with a95Sn/5Ag solder dot in the center and conductive adhesive on the nailhead.

Similar samples were subjected to a shear strength test conducted inaccordance with MIL-STD-202G, Method 211, Test Condition A, Procedure3.1.3 wherein the load applied is axial to the capacitor terminals andthe force is increased until the device fails. The results are providedin FIG. 21. In FIG. 21, Example 4 utilized a dot of polymer solder withSn95/Ag5 which was reflowed then bonded to a silver plated nail headwith conductive adhesive and post reflow cured. The control usedSn95/Ag5 solder and Example 5 used a conductive adhesive to bond to asilver plated lead wire. As demonstrated the conductive epoxy exhibiteda poor shear adhesion of less than 3 lbs. resulting in inadequatehandling strength for processing.

The inventive sample withstands greater than 400° C. with a 30 gramweight suspended from the external lead wire. Conductive adhesive alonesurvived >300° C. temperatures, but exhibits poor shear adhesion at roomtemperature as shown in FIG. 21. This is not acceptable for processingand handling of the part after joining such is common during assembly ofsubcomponents and electronic devices. Shear testing of the currentinvention showed an acceptable room temperature shear strength of >3lbs.

Example 9

Case size 2220, 0.47 μF, 50V rated MLCC with COG dielectric ceramicbased on calcium zirconate and nickel internal electrodes wheremanufactured by processes well known in the prior art. These wereterminated using a copper thick film paste containing a glass frit.Samples where made with two different types of electrolytic plating. Anickel plating was applied to the fired copper termination followed bycopper plating in one case and silver plating in the other. All theplating layer were done to a minimum of 5 microns (200 μinch). Leadlessstacks were produced for both MLCC plating types using a TLPS paste,Ormet CS510, containing mainly of copper and tin metal particles. Thestacks were manufactured by dispensing a thin bead of TLPS paste alongthe top surface of the plated terminations to be bonded. In this way4-chip stacks were assembled with the Ormet CS510 along the terminationsof adjacent capacitors. These were clamped in an assembly and heated topeak temperature of 330° C. remaining above 300° C. for 90 seconds usinga Heller oven under nitrogen atmosphere. Board flexure performance ofsamples of these leadless stacks were compared to the single MLCC byflexing to 10 mm using the test method described by AEC-Q200-005 Rev A.The flexure was applied at a rate of 1 mm/second with capacitance lossesof 2% being recorded as failures. The samples were connected to the testcircuit boards using reflowed tin-lead (SnPb) solder. These results areshown as Weibull graphs for the copper and silver plated parts in FIGS.22 and 23 respectively. Although the stack performance is slightly worsethan the single MLCC the flexure failures for leadless stacks made withboth types of plating are well above the 3 mm minimum required by AECfor COG type MLCC.

Example 10

Case size 2220, 0.50 μF, 500V rated MLCC with X7R dielectric ceramicbased on barium titanate and nickel internal electrodes wheremanufactured by processes well known in the prior art. These wereterminated using a copper thick film paste containing a glass frit.Samples were then plated with electrolytic nickel (minimum of 50 μinch)followed by tin (minimum of 100 μinch). Leadless stacks of 2 MLCC weremade using a TLPS paste, Ormet CS510, by dispensing a thin bead of TLPSpaste along the top surface of the plated terminations to be bonded.These were clamped in an assembly and heated to peak temperature of 330°C. remaining above 300° C. for 90 seconds using a Heller oven undernitrogen atmosphere. Board flexure performance of these leadless stackswere compared to the single MLCC by flexing to 10 mm using the testmethod described by AEC-Q200-005 Rev A. The flexure was applied at arate of 1 mm/second with capacitance losses of 2% being recorded asfailures. The samples were connected to the test circuit boards usingreflowed tin-silver-copper (SAC) solder. These results are shown asWeibull graphs in FIG. 26. The leadless stack performance is similar tothe single chip MLCC and both are well above the 2 mm minimum requiredby AEC for X7R type MLCC. FIG. 27 illustrates the flexure of a singlechip with a flex term to that of a 2 chip leadless stack with flex termand that the bulk of the distribution is well above 6 mm of flexure.

A further comparison between leaded stacks can be seen in example 11that compares high temperature capability of a control group of leadedstacks interconnected with SnSb solder with that of leaded stacksinterconnected with TLPS (CS510).

Example 11

Table 2 demonstrates the high temperature capability of the TLPSmaterial CS510 with that of the control group built with caps having thestandard Cu/Ni/Sn termination, a lead frame finish of Sn, and thecapacitor stack and lead frame terminated with standard SnSb solder. Thetest groups indicate the results of using various capacitor terminationmetallization's and different lead frame surface finishes as well as nocapacitor termination and using the CS510 TLPS interconnect to make boththe electrical and mechanical connection between the internal electrodesand the lead frame. It can be seen from Table 2 that the control groupwhen heated and suspended with a hanging 30 gram weight attached failedin a temperature range of 230 to 235 C. The samples made with OrmetCS510 regardless of capacitor termination metallization type includingthe non-terminated capacitor reached the test limit of 600 C and havingno failures. The only exception was the test group that used Sn on boththe termination and the lead frame surface which exhibited failures inthe range of 420-450° C.

TABLE 2 Comparative Hanging Weight Test Comparitive Hanging Weight TestResults Heated Hg. Wt. 30 g Part Test Samples Termination Lead FinishInterconnect Temp Fail? (° C.) 2220 504 500V Control Group Cu/Ni/Sn SnSnSb Solder 230-235 Failues Test Groups Cu/Ni/Sn Sn CS510 420, 450Failures Cu Frit Sn CS510 600 No Failures Cu CS510 600 No Failures AgCS510 600 No Failures Non Terminated Cu CS510 600 No Failures

Example 12

A comparison of the embodiment of this invention is illustrated in FIG.25 and compared to two plated coupons as illustrated in FIG. 24. In FIG.24 two copper coupons are illustrated with one plated with Ni and overplated with Ag and the second copper coupon plated with Ni andoverplated with Ag and then In. The two coupons were then placedface-to-face and heated to initiate the diffusion of indium. Afterprocessing, the two coupons were subjected to a shear test and pulleduntil joint failure occurred. The results demonstrate the need forintimate contact surface between the mating parts to ensure maximumjoint strength and uniform diffusion. The arrows in FIG. 24 indicatesisolated points of contact across the joint area where diffusion tookplace. The joint surface area is a 3.81×3.81 mm (0.150″×0.150″) squareor 14.52 mm² (0.0225 in²) and the joint shear strength of 266 psi.However, there is an estimated 20% surface contact area. This clearlydemonstrates the need for intimate contact between the mating surfacesin order to maximize joint strength.

FIG. 25 illustrates the same type of coupon as illustrated in FIG. 24,plated with 2.5 microns (100 micro-inch) of Ni and 5 microns (200micro-inch) of Ag and bonded to a second like coupon using In paste. Thesurface coverage was 100% and uniform and the shear strength recordedwas 9000 psi. The assembly, processing, and shear methods were exactlythe same thus demonstrating the difference of trying to bond twonon-planar surfaces together vs. bonding two non-planar surfacestogether using an Indium paste.

The invention has been described with reference to the preferredembodiments without limit thereto. One of skill in the art would realizeadditional embodiments and alterations which are not specifically setforth but which are within the scope of the claims appended hereto whichform an integral part of the instant application.

1-28. (canceled)
 29. A method of forming an electrical part comprising:providing multilayered ceramic capacitors wherein each multilayeredceramic capacitor of said multilayered ceramic capacitors comprisesfirst electrodes and second electrodes in an alternating stack with adielectric between each first electrode of said first electrodes and anadjacent second electrode of said second electrodes wherein said firstelectrodes terminate at a first side and said second electrodesterminate at a second side; stacking said multilayered ceramiccapacitors such that each said first side is parallel and each saidsecond side is parallel; forming a first layer of a first component of atransient liquid phase sintering conductive layer; forming a secondlayer of said first component of said transient liquid phase sinteringconductive layer; contacting said first layer and said second layer witha second component of said transient liquid phase sintering conductivelayer; heating to a first temperature sufficient to form a firsttransient liquid phase sintering conductive layer comprising said firstcomponent and said second component wherein said first transient liquidphase sintering conductive layer is in electrical contact with saidfirst electrodes and forming a second transient liquid phase sinteringconductive layer comprising said first component and said secondcomponent wherein said second transient liquid phase sinteringconductive layer is in electrical contact with said second electrodesthereby forming a stack capacitor.
 30. The method of forming anelectrical part of claim 29 further comprising forming an externaltermination between said first side and said first transient liquidphase sintering conductive layer on each said MLCC.
 31. The method offorming an electrical part of claim 30 wherein said first component isdiffused into each said external termination.
 32. The method of formingan electrical part of claim 21 wherein each said external terminationcomprises said second component.
 33. The method of forming an electricalpart of claim 32 further comprising forming a preform comprising saidfirst component.
 34. The method of forming an electrical part of claim33 wherein said preform is inserted between adjacent externalterminations prior to said heating.
 35. The method of forming anelectrical part of claim 34 wherein said preform further comprises asecond component.
 36. The method of forming an electrical part of claim34 wherein said preform further comprises an inert filler.
 37. Themethod of forming an electrical part of claim 36 wherein said inertfiller is glass frit.
 38. The method of forming an electrical part ofclaim 34 wherein said preform is malleable prior to said heating. 39.The method of forming an electrical part of claim 34 wherein saidpreform comprises particles wherein said particles comprise at least oneof said first component or said second component.
 40. The method offorming an electrical part of claim 39 wherein said particles comprise acore of said second component and a shell of said first component. 41.The method of forming an electrical part of claim 29 wherein said firstcomponent comprising at least one low melting point metal selected fromindium, tin, antimony, bismuth, cadmium, zinc, gallium, tellurium,mercury, thallium, selenium, polonium or lead.
 42. The method of formingan electrical part of claim 29 wherein said second component comprises ahigh temperature metal elected from the group consisting of silver,copper, aluminum, gold, platinum, palladium, beryllium, rhodium, nickel,cobalt, iron and molybdenum.
 43. The method of forming an electricalpart of claim 29 further comprising electrically connecting first leadsto said first electrodes with said first transient liquid phasesintering conductive layer between said first leads and said firstelectrodes.
 44. The method of forming an electrical part of claim 43wherein said first lead or said second lead comprises a materialselected from the group consisting of phosphor bronze, copper, andferrous alloys.
 45. The method of forming an electrical part of claim 43wherein said first lead or said second lead comprises a materialselected from the group consisting of alloys of copper, Alloy 42 andKovar.
 46. The method of forming an electrical part of claim 45 whereinsaid first lead or said second lead comprises a material selected fromthe group consisting beryllium copper, Cu194 and Cu192.
 47. The methodof forming an electrical part of claim 29 further comprising forming apreform comprising said first component and said second component. 48.The method of forming an electrical part of claim 47 wherein said secondcomponent comprises copper.
 49. A method of forming a stack ofmultilayered ceramic capacitors comprising: providing a multiplicity ofmultilayered ceramic capacitors wherein each multilayered ceramiccapacitor of said multilayered ceramic capacitors comprises: firstelectrodes and second electrodes in an alternating stack with adielectric between each first electrode of said first electrodes andeach adjacent second electrode of said second electrodes wherein saidfirst electrodes have a first polarity and terminate at a first side ofsaid multilayered ceramic capacitor and said second electrodes have asecond polarity and terminate at a second side of said multilayeredceramic capacitor; forming a stack of said multilayered ceramiccapacitors; forming a first transient liquid phase sintering conductivelayer in electrical contact with said first electrodes of adjacentmultilayered ceramic capacitors; and forming a second transient liquidphase sintering conductive layer in electrical contact with said secondelectrodes of adjacent multilayered ceramic capacitors.
 50. The methodof forming a stack of multilayered ceramic capacitors of claim 49further wherein said multilayered ceramic capacitors further comprise afirst termination in electrical contact with said electrodes and asecond termination in electrical contact with said second electrodes.51. The method of forming a stack of multilayered ceramic capacitors ofclaim 50 wherein said first transient liquid phase sintering conductivelayer is between adjacent first terminations.
 52. The method of forminga stack of multilayered ceramic capacitors of claim 51 wherein saidfirst transient liquid phase sintering conductive layer comprises a lowmelting point metal.
 53. The method of forming a stack of multilayeredceramic capacitors of claim 52 wherein said low melting point metal isselected from the group consisting of indium, tin, antimony, bismuth,cadmium, zinc, gallium, tellurium, mercury, thallium, selenium, orpolonium.
 54. The method of forming a stack of multilayered ceramiccapacitors of claim 49 wherein said forming said first transient liquidphase sintering conductive layer comprises placing a preform betweenadjacent multilayered ceramic capacitors.
 55. The method of forming astack of multilayered ceramic capacitors of claim 54 wherein saidpreform comprises a low melting point metal selected from the groupconsisting of indium, tin, antimony, bismuth, cadmium, zinc, gallium,tellurium, mercury, thallium, selenium, or polonium
 56. The method offorming a stack of multilayered ceramic capacitors of claim 54 whereinsaid preform is malleable.
 57. The method of forming a stack ofmultilayered ceramic capacitors of claim 54 wherein said preformcomprises a high melting point metal selected from the group consistingof silver, copper, lead, aluminum, gold, platinum, palladium, beryllium,rhodium, nickel, cobalt, iron and molybdenum.